The present invention relates generally to wireless communications systems and, in particular, to forward link power control in wireless communications systems.
Wireless communications systems use power control to improve system performance and increase system capacity. Power control involves tracking possible fading of communication channels and using the tracked fading to manage the power at which signals are being transmitted from base stations (in order to compensate for the fading). Prior art Code Division Multiple Access (CDMA) wireless communications systems based on the well-known IS-95 standard use error indicator bits to assist in controlling transmission power at the base station. Power control is implemented in the following manner.
When a call is set up in a prior art CDMA wireless communications system, a base station and a mobile-telephone communicate voice (or data) over a forward and a reverse link, wherein the forward link includes communication channels for transmitting signals from the base station to the mobile-telephone and the reverse link includes communication channels for transmitting signals from the mobile-telephone to the base station. Specifically, the base station transmits voice (or data) to the mobile-telephone over a communication channel referred to herein as a forward traffic channel, and the mobile-telephone transmits voice (or data) to the base station over a communication channel referred to herein as a reverse traffic channel. In either traffic channel, voice is transmit over 20 ms time intervals referred herein to as frames. Each forward traffic frame (i.e., frames transmitted over the forward traffic channel) includes voice (or data) and error control information in the form of a cyclical redundancy code (CRC). By contrast, each reverse traffic frame (i.e., frames transmitted over the reverse traffic channel) includes voice (or data) and error indicator bits (EIB) for indicating whether the last forward traffic frame is a good frame or in erasure, i.e., a bad frame.
When a base station transmits a forward traffic frame, a mobile-telephone receiving the forward traffic frame will check the CRC to determine whether the forward traffic frame is good or not. The mobile-telephone will indicate such determination to the base station using the EIB in the next reverse traffic frame the mobile-telephone will transmit. For example, a positive error indicator bit indicates no error in the forward traffic frame, and a negative error indicator bit indicates the forward traffic frame is a bad frame. Upon receiving reverse traffic frames from the mobile-telephone, the base station examines the error indicator bits and determines whether its forward link to the mobile-telephone is in fading, and adjusts the power of its forward link accordingly. For example, if the base station receive one or more successive error indicator bits denoting erred forward traffic frames, the base station may determine that its forward link is in fading and increase the power of its forward link.
In prior art CDMA wireless communications systems, a set of traffic channels (i.e., forward and reverse traffic channels) is used by the mobile-telephone to transmit either voice or data, but not both simultaneously, to and from the base station. In newly proposed CDMA (hereinafter referred to as CDMA 2000) wireless communications system, separate communication channels, referred to herein as forward and reverse fundamental and supplemental channels, may be used by the mobile-telephone to simultaneously transmit voice and data, respectively, to and from the base station. In other words, mobile-telephones using the CDMA 2000 wireless communications system may use multiple communication channels to transmit both voice and data at the same time. Accordingly, there exists a need for controlling power on multiple forward link communication channels, e.g., forward fundamental and supplemental channels.
The present invention is a method for controlling power on multiple forward link communication channels by using multiple power control sub-channels, wherein each power control sub-channel is associated with a forward link communication channel to be power controlled. In one embodiment, a fundamental power control sub-channel and a supplemental power control sub-channel are time multiplexed onto a reverse pilot channel, thus the reverse pilot channel comprises pilot, fundamental power control and supplemental power control sub-channels. Transmitted over the fundamental power control sub-channel is a fundamental power control bit for indicating to a base station to increase or decrease its transmission power over a corresponding forward fundamental channel. Transmitted over the supplemental power control sub-channel is a supplemental power control bit for indicating to a base station to increase or decrease its transmission power over a corresponding forward supplemental channel. Each pilot control sub-channel may be separated by, at least one power sub-channel for time diversity purposes in order to better combat fading of the pilot channel.